1. Field of the Invention
This invention relates to a video display control system adapted to be connected to a video display unit such as a video monitor for displaying a video image on a screen of the video display unit.
2. Prior Art
In recent years, video display control systems capable of displaying both of still and animation pattern images on a screen of a video display unit have been extensively used in graphic video display apparatuses such as video game machines. One example of such conventional systems is shown in U.S. Pat. No. 4,286,320. This conventional system comprises a video data memory with an auto-incrementing address counter which automatically increments the contents thereof when a transfer of data is effected between a central processing unit and the video data memory. However, with this conventional system, when it is required to transfer a block of video data which represent, for example, an image composed of a plurality of rows of display elements in an area on the screen, the address count must be preset to an address corresponding to the first display element of the next row each time a transfer of video data of the current row is completed. A program to be executed by the central unit to implement the above procedure is fairly complicated. Furthermore, the address data to be outputted from the central processing unit is not a data indicative of the position of the display element on the screen but is a data indicative of an actual address or a memory location in the video data memory. Thus, it is rather difficult to recognize the position of a dot on the screen from the address data outputted from the central processing unit to display it. And, if such data indicative of the position of display element on the screen is used, it is necessary for a central processing unit 2 to convert the data into an actual address of a memory location corresponding to the data. As a result, the processing by the central processing unit becomes more complicated. Another example of the conventional video display control systems will be described below. This conventional system is capable of displaying a variety of patterns as a still image on the screen. However, the still image displayed in this system is nothing but a combination of selected ones of a predetermined number (for example, 256) of patterns previously stored in the video data memory, each of the patterns being composed of, for example, 8.times.8 display elements or dots. And therefore, this conventional system can not display a very complicated still image and can not also display even a simple still image in some cases. For example, it is often desired for this kind of display control system to display a still image with a moving line on the screen, however, with this conventional system, it has been impossible to display such a still image. The reason for this is that, to display a still image with a moving line on the screen, the patterns in the video data memory and the combination thereof must be changed by the central processing unit at a high rate. Also, it is often required for this kind of system to display on the screen a still image with a complicated line such as a circle, however, with the conventional system, such a still image can not have been displayed, since the conventional system has not been capable of displaying a dot at a desired position on the screen. Furthermore, this conventional system can not paint out a selected display area in a still image on the screen in a rapid manner. The reason for this is that the conventional system has not been capable of detecting a boundary between an area on the screen and areas surrounding it, such a detection being essential to a paint-out of an area on the screen. And therefore, the central processing unit has required much time to execute a program for such a detection. Further examples of the conventional video display systems are shown in U.S. Pat. Nos. 4,243,984, 4,262,302, 4,374,395 and 4,387,406, however none of them have overcome the above-described deficiencies of the conventional video display control systems.